1. Field of the Invention
The invention relates to a Cu—Ni—Mo alloy thin film and method for manufacturing the same.
2. Description of the Related Art
Cu possesses good electric conductivity and resistivity against electromigration, and is used as interconnect metal for ultra-large scale integrated circuits. However, Cu reacts easily with Si or silica at low temperatures about 200° C. To avoid device failure of this kind, a diffusion barrier layer should be introduced to inhibit the inter-diffusion. With the ever decreasing device size, the barrier layer thickness should be reduced consequently. In accordance with international semiconductor roadmap, for 32 and 22 nm interconnect techniques, the barrier thickness should be 7 and 4 nm only. Conventional diffusion barrier layer of such a thickness faces a big challenge to maintain the barrier property and a high thermal stability.
Alternatively, the Cu seed layer can be alloyed for such a purpose, avoiding the use of barrier layer technique. Properly selecting the elements and their quantities might retard Cu—Si reaction and thus minimize the rise of electrical resistivity due to alloying.
Early research chose doping elements that have strong chemical affinity with oxygen such as Mg and Al. After annealing, a thin passivation layer is formed on the film and at the interface. However such a passivation layer is rather thick, about 20 nm, which is not suitable for Cu interconnect down to 45 nm
There are two major problems in the present Cu alloys for barrierless interconnect.
1. The doping involves mainly insoluble elements of large atomic sizes. Though the doped elements are prone to precipitate and effectively decrease electrical resistivity, the secondary phase precipitation stabilizes the defect structures in the as-deposited state that increases electrical scattering. This is why the film resistivity cannot be decreased to the desired level.
2. The doping quantity is not known. Too small doping cannot effectively enhance diffusion barrier; excessive doping increases resistivity significantly. Adding two or more elements has not been tried, and a theoretical guidance is quite needed.